Izod Impact: ISO vs ASTM

Sorry for all of the math in this post

If you are unaware, there are two sets of test methods that are currently in use for many of the common tests that we see reported on data sheets and certificates of analysis. This has caused a lot of confusion.

To add to the confusion, I have seen many data sheets and certificates of analysis in which data is reported incorrectly. None more than izod impact. There is currently an ASTM method and an ISO method for testing izod impact. Although some data sheets claim that they are, the two test methods used for izod impact are definitely not equivalent. They are two different test methods and there is no way of converting between the two.

In the mid 1990s, the International Organization for Standardization that writes the ISO 9000 standards started developing test methods for materials. These methods were similar to the ASTM methods that we were familiar with and also similar to DIN methods that were used in Europe.

They tried to make sure that the new test methods could be adopted relatively easily by laboratories using existing laboratory equipment with little or no modification. The ISO test methods did however require new test specimens. These test specimens had metric instead of imperial (inches) dimensions.

Some of the ISO test methods, like tensile properties are actually considered to be equivalent to their ASTM counterparts. Most of the test methods are not though. The ASTM method for izod Impact is D 256. The ISO method is ISO 180.

Izod impact works like this: A test plaque is molded. In most cases, a small notch is cut into the test specimen to make it more likely to fracture and give a meaningful result. The test specimen is then clamped into a vice in a pendulum impact tester. The machine has a weight that swings and impacts the specimen. The energy required to break the specimen is recorded. Because the pendulum swings in a circle, the energy is measured as torque similar to a torque measurement on a car engine.

Pendulum Impact Tester Used for Izod

The torque is then divided by the size of the specimen to give a result that states torque per specimen size. The specimen size must be taken into account because otherwise a ½ inch thick specimen would show a much higher Izod impact than a specimen measuring 1/8 inches thick. Dividing the torque by the specimen size compensates for the size.

There are two primary differences between the ASTM method and the ISO method. One is that the ASTM uses a specimen that is ½ inch wide and can be either 1/8 or 1/4 inches thick. The ISO method uses a specimen that is 10mm wide by 4 mm thick. The second and biggest difference is that the ASTM method requires you to take the torque and divide it by the specimen thickness only. The ISO method requires you to divide the torque by the thickness times the width of the specimen, in other words, the cross-sectional area.¹

Notched ASTM Test Specimen

With the ASTM method, you have a torque/thickness value. With the ISO method, you have a torque/cross-sectional area value. You see why these cannot be converted from one to the other. You cannot convert these any more than you can convert a length to an area. Go to the carpet store and tell them that your room is 10 feet long and ask them how much carpet that you need. They need to know the length and width of the room in order to figure the area.

You will typically see the ASTM method izod reported in units of ft-lb/in or J/m. The ISO method is usually reported in kJ/m² (notice the squared) or less commonly ft-lb/in².

Conversions for the different units used for izod impact:

ASTM method:

If you know J/m and want ft-lbs/in, then multiply by 0.0187

ISO method:

If you know ft-lb/in² and want kJ/m², then multiply by 2.1

If you take away one thing from this article, it would be that any time you see a square at the end of the unit; it is the ISO test method. If you do not, it is the ASTM method.

Comparison of izod impacts for ASTM method vs ISO method for some common materials.

Material ASTM D 256 (notched) ISO 180 (notched)

PPCO Impact 2.5 ft-lb/in 13 kJ/m²

Nylon 6/6 unfilled 1.0 ft-lb/in 5.5 kJ/m²

ABS 3.2 ft-lb/in 16 kJ/m²

PC 15 ft-lb/in 69 kJ/m²

Correlations like this can be useful for making rough estimations of impact strength results but the only way to determine the impact strength is to perform the testing using the method that you need.

It should also be noted that both methods specify that results are reported to two significant figures. I often see results listed in more significant figures than are allowed by the test method. The problem with this is that the extra significant figures overstate the accuracy of the test method. If the calculation reveals an Izod impact of 3.22 ft-lb/in, this should be rounded to 3.2 because the additional 0.02 is considered to be beyond the capability of the test method.

Having two different sets of test methods that are both in common use today makes it difficult to compare materials. I don’t know, maybe the material manufacturers like it that way. It appears to me that most nylon manufacturers show ISO methods on their data sheets while most other materials seem to rely on ASTM methods. I noticed on Prospector that some manufacturers like SABIC show different data sheets for North America, Europe and Asia. The North American data sheets typically report ASTM data while the European data sheets typically show ISO methods. The Asian data sheets tend to use ASTM methods for reasons that are unclear.

I hope this article clarifies this a bit for you. Judging by some of the data sheets that I have seen, a lot of people are confused by this.

Note 1: When performing either test method, a micrometer is used to take actual measurements of each test specimen. The test specimen dimensions specified by the test methods are the dimensions of the mold cavity. The test specimens will be slightly smaller because of shrink.